The term “molecular electronics” has been used to describe phenomena or devices that include a molecule as a circuit element. The motivation for the field is the prospect of making extremely small (potentially one molecule) electronic components with a much wider range of functions than conventional semiconductor electronic devices. If molecular devices become practical, a wide variety of applications in microelectronics, computing, imaging and display technology and chemical sensing can be envisioned.
The large majority of proposed molecular electronic devices are based on the gold-thiol system (Au/thiol), in which organic mercaptans “self assemble” on a flat gold surface to form an ordered monomolecular layer. In many cases, scanning tunneling microscopy (STM) of the Au/thiol layer or of gold particles thereon reveal the electron transfer characteristics of the monolayer molecule. However, the Au/thiol system is not suitable for the manufacture of stable devices in a highly parallel fashion since the thiol molecules maintain some mobility on the Au surface and are sensitive to elevated temperatures.
An alternative approach involves placing a single layer of molecules between two metal or metal oxide surfaces using Langmuir-Blodgett technology. In these experiments, the current/voltage behavior of a layer of molecules may be obtained, with the current path extending through the molecule itself.
While the above experiments demonstrate certain characteristics of molecules as electronic components, they have severe disadvantages when considered for practical uses. First, the apparatus required is extremely complex and difficult to use, and so far has only been successfully implemented on a limited scale in very sophisticated laboratories. Second, both approaches result in films with unavoidable defects that limit both the size and lifetime of the devices. Third, the Au/thiol or Langmuir-Blodgett approaches to binding the molecule to two conductors generate large energy barriers that reduce current flow. In the former, the aliphatic molecules have large electron transfer barriers and the dipole formed between the gold surface and sulfur atom can decrease electronic coupling between conductor and molecule. The Langmuir-Blodgett approach often requires metal oxide films that purposely decouple the molecule from the conductor in terms of electronic interactions. When the molecule is electronically decoupled from the conductor, many potentially valuable applications of molecular electronics are prevented. In addition, Langmuir-Blodgett structures involve weak molecule-to-surface bonds, and are thermally quite fragile. Fourth, STM interrogates one or a few molecules at a time, so the massive parallelism inherent in microelectronic devices is difficult to conceive. These fundamental problems inherent in Au/thiol or Langmuir Blodgett devices prevent any conceivable practical application in the foreseeable future.
Thus, significant improvements are needed in the evolution of molecular electronic device technology.